The subject matter described herein relates to vehicle exhaust systems, and in particular to the management of temperatures within such systems between engine shut-down and subsequent start-up as well as during operation.
Producing vehicles capable of meeting ever tightening emissions regulations presents challenges to manufacturers. A major component in the conversion of unwanted CO and NOx gases as well as unburned fuel to more environmentally benign chemical species is the catalytic converter system. Catalytic converters contain one or more catalyst bricks, typically in the form of a channeled substrate material such as cordierite. The substrate is wash coated with catalytically active precious metals and metal support materials such as alumina or cerium zirconium oxide.
Several technical issues exist with current catalytic converters that affect the devices' ability to operate at optimum efficiency. A first issue is that in order to operate effectively, the catalyst needs to be at elevated temperatures (above about 700° C. for a gasoline fueled vehicle and above 200° C. for a diesel vehicle). Thus, when a cold internal combustion or diesel engine with a catalytic converter is started, the emission of pollutants is high, as the catalyst within the catalytic converter does not function at low temperatures. The exhaust emitted at start-up heats the exhaust manifold and the exhaust pipe before heating the catalytic converter. It may take several minutes for the cold catalytic converter to be heated to “light off” temperature. The “light off” temperature is the temperature at which the catalytic converter oxidizes at least fifty percent of hydrocarbons in engine exhaust. It has been reported that 60 to 80 percent of all hydrocarbon emissions occur during the first few minutes after engine startup.
To reduce the emission of pollutants at startup, efforts have been directed at maintaining the catalytic converter at a functional temperature using a variety of both active and passive techniques including fuel combustion, preheating the catalytic converter, rapidly heating the catalytic converter after startup using electrical heating, or using an increased fuel to air ratio. Other efforts have involved absorbing and storing pollutants in zeolites until the catalytic converter has reached a functional temperature. However, such efforts have led to systems that are both conceptually and mechanically complex, requiring added components which add to the cost and complexity of manufacturing.
Another issue is aging of the catalyst in the converter. The aging process can include degradation of inactive materials in the converter and sintering of the finely distributed metal catalyst particles which reduces their surface area and hence their catalytic effect. These changes are initiated and/or greatly accelerated at very high temperatures above about 950° C. that can occur in heavy load operation or during a period of frequent misfire of the engine. The chemical reactions that occur in the catalyst system can also be highly exothermic and can easily raise temperatures into a range that damages the catalyst. Ideally, a catalyst system would include some means of temperature regulation, both to keep the catalyst warm when the engine is shut off for short periods (especially important for hybrid vehicles where the engine will regularly turn on and off) as well as to prevent overheating of temperature-sensitive and expensive catalyst materials.
Thus, efforts have also been made to control catalytic converter temperature during engine operation. Aspects of the technology for controlling catalytic converter temperature during engine operation are related to maintaining the catalytic converter at functional temperatures between engine uses. Some of these efforts have utilized phase change materials (“PCM”) to store heat energy and inhibit heat loss during engine start-up and to absorb heat during engine operation to prevent overheating of the converter. For example, U.S. Pat. No. 5,477,676 describes a catalytic converter surrounded by variable conductance insulation that includes thermal storage media in the form of phase change materials. Typical of such systems is the presence of vacuum sealed chambers, shrouds, and jackets for containing the phase change materials when they melt. Again, the need for vacuum contained devices adds to the complexity and costs of manufacturing such catalytic converter systems.
Accordingly, the need still exists in the art for vehicle exhaust systems utilizing catalytic converters which are capable of maintaining the temperature of the catalyst between engine shut-down and subsequent start-up as well as regulating the temperature of the catalysts during engine operation, and yet which are simple in design and manufacture.